Electromyography system

ABSTRACT

A system and methods that can be used to monitor and detect nerve status and/or proximity by determining relative neuro-muscular response onset value thresholds for a nerve. The nerve status and/or proximity may be determined by delivering and increasing a stimulation signal to depolarize the nerve and measuring the current intensity level at which a neuro-muscular response to the depolarization of the nerve is detected.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a division of commonly owned and U.S. patentapplication Ser. No. 09/722,070 filed Nov. 24, 2000, now U.S. Pat. No.7,470,236, which claims benefit under 35 U.S.C. §119(e) from U.S.Provisional Application Ser. No. 60/167,416 filed Nov. 24, 1999, theentire contents of which are all hereby expressly incorporated byreference into this disclosure as if set forth fully herein.

TECHNICAL FIELD

The present invention relates to electromyography (EMG) and to systemsfor detecting the presence of nerves during surgical procedures.

BACKGROUND OF THE INVENTION

It is important to avoid unintentionally contacting a patient's nerveswhen performing surgical procedures, especially when using surgicaltools and procedures that involve cutting or boring through tissue.Moreover, it is especially important to sense the presence of spinalnerves when performing spinal surgery, since these nerves areresponsible for the control of major body functions. However, avoidinginadvertent contact with these nerves is especially difficult due to thehigh nerve density in the region of the spine and cauda equina.

The advent of minimally invasive surgery offers great benefits topatients through reduced tissue disruption and trauma during surgicalprocedures. Unfortunately, a downside of such minimally invasivesurgical procedures is that they tend to offer a somewhat reducedvisibility of the patient's tissues during the surgery. Accordingly, thedanger of inadvertently contacting and/or severing a patient's nervescan be increased.

Systems exist that provide remote optical viewing of a surgical siteduring minimally invasive surgical procedures. However, such systemscannot be used when initially penetrating into the tissue. Moreover,such optical viewing systems cannot reliably be used to detect thelocation of small diameter peripheral nerves.

Consequently, a need exists for a system that alerts an operator that aparticular surgical tool, which is being minimally invasively insertedinto a patient's body, is in close proximity to a nerve. As such, theoperator may then redirect the path of the tool to avoid inadvertentcontact with the nerve. It is especially important that such a systemalerts an operator that a nerve is being approached as the surgical toolis advanced into the patient's body prior to contact with the nerve,such that a safety distance margin between the surgical tool and thenerve can be maintained.

A variety of antiquated, existing electrical systems are adapted tosense whether a surgical tool is positioned adjacent to a patient'snerve. Such systems have proven to be particularly advantageous inpositioning a hypodermic needle adjacent to a nerve such that the needlecan be used to deliver anesthetic to the region of the body adjacent thenerve. Such systems rely on electrifying the needle itself such that asa nerve is approached, the electrical potential of the needle willdepolarize the nerve causing the muscle fibers coupled to the nerve tocontract and relax, resulting in a visible muscular reaction, seen as a“twitch”.

A disadvantage of such systems is that they rely on a visual indication,being seen as a “twitch” in the patient's body. During precisionminimally invasive surgery, uncontrollable patient movement caused bypatient twitching, is not at all desirable, since such movement mayitself be injurious. In addition, such systems rely on the operator tovisually detect the twitch. Accordingly, such systems are quite limited,and are not particularly well adapted for use in minimally invasivesurgery.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for informing anoperator that a surgical tool or probe is approaching a nerve. Inpreferred aspects, the surgical tool or probe may be introduced into thepatient in a minimally invasive cannulated approach. In alternateaspects, the surgical tool or probe comprises the minimally invasivecannula itself.

In a first aspect, the present invention provides a system for detectingthe presence of a nerve near a surgical tool or probe, based upon thecurrent intensity level of a stimulus pulse applied to the surgical toolor probe. When a measurable neuro-muscular (EMG) response is detectedfrom a stimulus pulse having a current intensity level at or below apre-determined onset level, the nerve is considered to be near the toolor probe and thus, detected.

In an optional second aspect of the invention, the onset level (i.e.:the stimulus current level at which a neuro-muscular response isdetected for a particular nerve) may be based on EMG responses measuredfor a probe at a predetermined location relative to the nerve.Specifically, onset levels may first be measured for each of a pluralityof spinal nerves, (yielding an initial “baseline” set of neuro-muscularresponse onset threshold levels), which are then used in the first(nerve detection) aspect of the invention. Therefore, in accordance withthis optional second aspect of the invention, a system for determiningrelative neuro-muscular onset values (i.e.: EMG response thresholds),for a plurality of spinal nerves is also provided. Accordingly, thepre-determined onset level may be compared to the current level requiredto generate a measurable EMG response for a tool or probe being advancedtoward one or more nerves of interest.

In alternate aspects, however, the neuro-muscular onset values that areused to accomplish the first (nerve detection) aspect of the inventionare not measured for each of the patient's plurality of spinal nerves.Rather, pre-determined levels of current intensity (below whichneuro-muscular responses are detected in accordance with the firstaspect of the invention) can instead be directly pre-set into thesystem. Such levels preferably correspond to specific expected ordesired onset threshold values, which may have been determinedbeforehand by experimentation on other patients.

In the aspect of the invention where initial “baseline” neuro-muscularonset values are determined prior to nerve detection, such onset valuescan optionally be used to calibrate the present nerve-detection system(which in turn operates to detect whether an minimally invasive surgicaltool or probe is positioned adjacent to a spinal nerve).

It is to be understood, therefore, that the present invention is notlimited to systems that first determine relative neuro-muscular onsetvalues, and then use these neuro-muscular onset values to detect thepresence of a nerve. Rather, the present invention includes an optionalsystem to first determine relative neuro-muscular onset values and asystem to detect the presence of a nerve (using the neuro-muscular onsetvalues which have been previously determined). As such, the presentinvention encompasses systems that also use fixed neuro-muscular onsetvalues (which may simply be input into the system hardware/software bythe operator prior to use) when performing electromyographic monitoringof spinal nerves to detect the presence of a spinal nerve adjacent atool or probe.

In optional aspects, the preferred method of sensing for the presence ofa nerve may be continuously repeated as the probe/surgical tool isphysically advanced further into the patient such that the operator iswarned quickly when the probe/surgical tool is closely approaching thenerve.

In the first (nerve sensing) aspect of the invention, the presentnerve-detection system comprises an electrode or electrodes positionedon the distal end of the surgical tool or probe, with anelectromyographic system used to detect whether a spinal nerve ispositioned adjacent to the surgical tool or probe. A conclusion is madethat the surgical tool or probe is positioned adjacent to a spinal nervewhen a neuro-muscular (e.g.: EMG) response to a stimulus pulse emittedby the electrode or electrodes on the surgical tool or probe is detected(at a distant myotome location, such as on the patient's legs) at orbelow certain neuro-muscular response onset values (i.e.: pre-determinedcurrent intensity levels) for each of the plurality of spinal nerves.The stimulus pulse itself may be emitted from a single probe, but in anoptional aspect, the stimulus pulse may be emitted from separate leftand right probes with the signals being multiplexed. As stated above,such pre-determined levels may be pre-input by the operator (or bepre-set into the system's hardware or software) and may thus optionallycorrespond to known or expected values. (For example, values as measuredby experimentation on other patients).

In accordance with the optional second (neuro-muscular response onsetvalue determination) aspect of the invention, the neuro-muscularresponse onset values used in nerve detection may instead be measuredfor the particular patient's various nerves, as follows.

Prior to attempting to detect the presence of a nerve, an EMG stimuluspulse is first used to depolarize a portion of the patient's caudaequina. This stimulus pulse may be carried out with a pulse passingbetween an epidural stimulating electrode and a corresponding skinsurface return electrode, or alternatively, between a pair of electrodesdisposed adjacent to the patient's spine, or alternatively, oralternatively, by a non-invasive magnetic stimulation means. It is to beunderstood that any suitable means for stimulating (and depolarizing aportion of) the patient's cauda equina can be used in this regard.

After the stimulus pulse depolarizes a portion of the patient's caudaequina, neuro-muscular (i.e., EMG) responses to the stimulus pulse arethen detected at various myotome locations corresponding to a pluralityof spinal nerves, with the current intensity level of the stimulus pulseat which each neuro-muscular response is first detected being theneuro-muscular response “onset values” for each of the plurality ofspinal nerves.

It is to be understood that the term “onset” as used herein is notlimited to a condition in which all of the muscle fibers in a bundle ofmuscle fibers associated with a particular nerve exhibit aneuro-muscular response. Rather, an “onset” condition may comprise anypre-defined majority of the muscle fibers associated with a particularnerve exhibit a neuro-muscular response.

In an additional aspect of the invention, the relative neuro-muscularresponse onset values can be repeatedly re-determined (at automaticintervals or at intervals determined by the operator) so as to accountfor any changes to the response onset values caused by the surgicalprocedure itself. Accordingly, a further advantage of the presentinvention is that it permits automatic re-assessment of the nervestatus, with the relative neuro-muscular response onset values for eachof the plurality of spinal nerves being re-determined before, during andafter the surgical procedure, or repeatedly determined again and againduring the surgical procedure. This optional aspect is advantageousduring spinal surgery as the surgery itself may change the relativeneuro-muscular response onset values for each of the plurality ofnerves, such as would be caused by reducing the pressure on an exitingspinal nerve positioned between two adjacent vertebrae. This periodicre-determination of the onset values can be carried out concurrentlywith the nerve sensing function.

Accordingly, an advantageous feature of the present invention is that itcan simultaneously indicate to an operator both: (1) nerve detection(i.e.: whether the surgical tool/probe is near a nerve); and (2) nervestatus changes (i.e.: the change in each nerve's neuro-muscular responseonset values over time). The surgeon is thus able to better interpretthe accuracy of nerve detection warnings by simultaneously viewingchanges in the various onset levels. For example, should the surgeonnote that a particular onset value (i.e.: the current level of astimulus pulse required to elicit an EMG response for a particularnerve) is increasing, this would tend to show that this nerve pathway isbecoming less sensitive. Accordingly, a “low” warning may be interpretedto more accurately correspond to a “medium” likelihood of nerve contact;or a “medium” warning may be interpreted to more accurately correspondto a “high” likelihood of nerve contact.

Optionally, such re-assessment of the nerve status can be used toautomatically re-calibrate the present nerve detection system. This canbe accomplished by continually updating the onset values that are thenused in the nerve detection function.

In preferred aspects, the neuro-muscular response onset values for eachof the plurality of spinal nerves are measured at each of thespaced-apart myotome locations, and are visually indicated to anoperator (for example, by way of an LED scale). Most preferably, themeasuring of each of the various neuro-muscular response onset values isrepeatedly carried out with the present and previously measured onsetvalue levels being simultaneously visually indicated to an operator suchas by way of the LED scale.

Accordingly, in one preferred aspect, for example, different LED lightscan be used to indicate whether the value of each of the variousneuro-muscular response onset values is remaining constant over time,increasing or decreasing. An advantage of this optional feature of theinvention is that a surgeon operating the device can be quickly alertedto the fact that a neuro-muscular response onset value of one or more ofthe spinal nerves has changed. Should the onset value decrease for aparticular nerve, this may indicate that the nerve was previouslycompressed or impaired, but become uncompressed or no longer impaired.

In a particular preferred embodiment, example, a blue LED can be emittedat a baseline value (i.e.: when the neuro-muscular response onset valueremains the same as previously measured); and a yellow light can beemitted when the neuro-muscular response onset value has increased fromthat previously measured; and a green light being emitted when theneuro-muscular response onset value has decreased from that previouslymeasured.

In an alternate design, different colors of lights may be simultaneouslydisplayed to indicate currently measured onset values for each of theplurality of spinal nerve myotome locations, as compared to previouslymeasured onset values. For example, the present measured onset valuelevels for each of the plurality of spinal nerve myotome locations canappear as yellow LED lights on the LED scale, with the immediatelypreviously measured onset value levels simultaneously appearing as greenLED lights on the LED scale. This also allows the operator to comparepresently measured (i.e. just updated) neuro-muscular response onsetvalues to the previously measured neuro-muscular response onset values.

In preferred aspects, the present system also audibly alerts theoperator to the presence of a nerve. In addition, the volume orfrequency of the alarm may change as the probe/tool moves closer to thenerve.

In a preferred aspect of the present invention, the neuro-muscular onsetvalues, (which may be detected both when initially determining therelative neuro-muscular response onset values in accordance with thesecond aspect of the invention, and also when detecting a neuro-muscularonset response to the emitted stimulus pulse from the probe/tool inaccordance with the first aspect of the invention), are detected bymonitoring a plurality of distally spaced-apart myotome locations whichanatomically correspond to each of the spinal nerves. Most preferably,these myotome locations are selected to correspond to the associatedspinal nerves that are near the surgical site. Therefore, these myotomelocations preferably correspond with distally spaced-apart on thepatient's legs (when the operating site is in the lower vertebralrange), but may also include myotome locations on the patient's arms(when the operating site is in the upper vertebral range). It is to beunderstood, however, that the present system therefore encompassesmonitoring of any relevant myotome locations that are innervated bynerves in the area of surgery. Therefore, the present invention can beadapted for use in cervical, thoracic or lumbar spine applications.

During both the optional initial determination of the relativeneuro-muscular response onset values for each of the plurality of spinalnerves (i.e.: the second aspect of the invention) and also during thedetection of neuro-muscular onset responses to the stimulus pulse fromthe surgical probe/tool (i.e.: the first aspect of the invention), theemission of the stimulus pulse is preferably of a varying currentintensity. Most preferably, the stimulus pulse is incrementallyincreased step-by-step in a “staircase” fashion over time, at leastuntil a neuro-muscular response signal is detected. The stimulus pulseitself may be delivered either between a midline epidural electrode anda return electrode, or between two electrodes disposed adjacent thepatient's spine, or from an electrode disposed directly on theprobe/tool, or by other means.

An important advantage of the present system of increasing the level ofstimulus pulse to a level at which a response is first detected is thatit avoids overstimulating a nerve (which may cause a patient to“twitch”), or cause other potential nerve damage.

In optional preferred aspects, the “steps” of the staircase ofincreasing current intensity of the stimulus pulse are carried out inrapid succession, most preferably within the refractory period of thespinal nerves. An advantage of rapidly delivering the stimulus pulseswithin the refractory period of the spinal nerves is that, at most, onlya single “twitch” will be exhibited by the patient, as opposed to amuscular “twitching” response to each level of the stimulation pulse aswould be the case if the increasing levels of stimulus pulse wereinstead delivered at intervals of time greater than the refractoryperiod of the nerves.

In another optional preferred aspect, a second probe is added to thepresent system, functioning as a “confirmation electrode”. In thisoptional aspect, an electrode or electroded surface on the second probeis also used to detect the presence of a nerve, (using the same systemas was used for the first probe to detect a nerve). Such a second“confirmation electrode” probe is especially useful when the first probeis an electrified cannula itself, and the second “confirmationelectrode” probe is a separate probe that can be advanced through theelectrified cannula. For example, as the operating (electrified) cannulais advanced into the patient, this operating cannula itself functions asa nerve detection probe. As such, the operating cannula can be advancedto the operating site without causing any nerve damage. After thiscannula has been positioned at the surgical site, it may then be used asthe operating cannula through which various surgical tools are thenadvanced. At this stage, its nerve-sensing feature may be optionallydisabled, should this feature interfere with other surgical tools orprocedures. Thereafter, (and at periodic intervals, if desired) thesecond “confirmation electrode” probe can be re-advanced through theoperating cannula to confirm that a nerve has not slipped into theoperating space during the surgical procedure. In the intervals of timeduring which this second “confirmation electrode” probe is removed fromthe operating cannula, access is permitted for other surgical tools andprocedures. The second “confirmation electrode” probe of the presentinvention preferably comprises a probe having an electrode on its distalend. This confirmation electrode may either be mono-polar or bi-polar.

In an optional preferred aspect, the second “confirmation electrode”probe may also be used as a “screw test” probe. Specifically, theelectrode on the secondary “confirmation” probe may be placed in contactwith a pedicle screw, thereby electrifying the pedicle screw. Should thepresent invention detect a nerve adjacent such an electrified pediclescrew, this would indicate that pedicle wall is cracked (since theelectrical stimulus pulse has passed out through the crack in thepedicle wall and stimulated a nerve adjacent the pedicle).

An advantage of the present system is that it may provide both nerve“detection” (i.e.: sensing for the presence of nerves as the probe/toolis being advanced) and nerve “surveillance” (i.e.: sensing for thepresence of nerves when the probe/tool had been positioned).

A further important advantage of the present invention is that itsimultaneously monitors neuro-muscular responses in a plurality ofdifferent nerves. This is especially advantageous when operating in theregion of the spinal cord due to the high concentration of differentnerves in this region of the body. Moreover, by simultaneouslymonitoring a plurality of different nerves, the present system can beused to indicate when relative nerve response onset values have changedamong the various nerves. This information can be especially importantwhen the surgical procedure being performed can alter the relative nerveresponse onset value of one or more nerves with respect to one another.

A further advantage of the present system is that a weaker currentintensity can be applied at the nerve detecting electrodes (on theprobe) than at the stimulus (i.e.: nerve status) electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of various components of the present inventionin operation.

FIG. 2 shows a current intensity staircase for an electromyographicstimulation (nerve status) electrode.

FIG. 3 shows a current intensity staircase for an electromyographicstimulation pulse for a nerve detection electrode disposed on a probe.

FIG. 4 corresponds to FIG. 1, but also shows exemplary “high”, “medium”and “low” warning levels corresponding to exemplary neuro-muscularresponse onset levels.

FIG. 5 shows a patient's spinal nerves, and corresponding myotomemonitoring locations.

FIG. 6 is an illustration of the waveform characteristics of a stimuluspulse and a corresponding neuro-muscular (EMG) response as detected at amyotome location.

FIG. 7 is a schematic diagram of a nerve detection system.

FIG. 8A is an illustration of the front panel of one design of thepresent nerve status and detection system.

FIG. 8B is an illustration of the front panel of another design of thepresent nerve status and detection system.

FIG. 9 is a flow chart illustrating an example method for determining atleast one of a change in nerve status and a change in nerve proximity,according to one example embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention sets forth systems for detecting when a nerve isnear or adjacent to an electrified surgical tool, probe, cannula, orother surgical instrument. The present invention also involves optionalsystems for simultaneously determining the “status” (e.g.: sensitivity)of a plurality of nerves.

As will be explained, the present system involves applying a signal witha current level to a probe near a nerve and determining whether anelectromyographic “EMG” (i.e.: neuro-muscular) response for a musclecoupled to the nerve is present.

In preferred aspects, the present system applies a signal with a knowncurrent level (mA) to a “probe” (which could be midline probe, acannula, a needle, etc.) Depending on the current level, distance to thenerve, and health of the nerve, an EMG may be detected in a musclecoupled to the nerve. In accordance with preferred aspects, an EMGresponse is determined to have been detected when the peak-to-peakresponse of the EMG signal is greater than some level (mVolts). In otherwords, an EMG response is determined to have been detected when thestimulus current level generates an EMG having a peak-to-peak valuegreater than a pre-determined level (for example, 60 mV or 80 mV inspinal nerve applications.) Such stimulus current level at which an EMGresponse is detected is termed the “onset” current level for the nerve.

In optional aspects, the present invention also sets forth systems fordetermining these onset current values (i.e.: determining the stimuluscurrent level at which an EMG response is detected with a maximumpeak-to-peak value greater than a predetermined level). Such onsetvalues may be determined for a plurality of nerves either in absoluteterms, or in relation to one another.

The first aspect of the present invention involves nerve detection. Inthe optional second aspect of the invention, nerve status informationmay be used to aid nerve detection. The nerve status aspect determinesthe minimum current level of a signal applied to a probe near a nerveneeded to generate onset EMG response for a muscle coupled to a nerve ofinterest. The present invention may use this determined minimum currentlevel when determining whether a probe is near the same nerve.

In optional aspects, the present invention may involve determining aninitial set of “baseline” neuro-muscular response onset values for aplurality of different spinal nerve pathways. This optional second(nerve status) aspect of the present invention is preferably carried outprior to the first (nerve detection) aspect of the invention, with theinitial set of “baseline” neuro-muscular onset values then optionallybeing used in the nerve detection function, as will be explained below.As the optional second aspect of the invention is carried out prior tocarrying out the first aspect of the invention, it will be describedfirst.

In the nerve status determination, the minimum current level of a signalapplied to a probe needed to generate an onset neuro-muscular response(i.e.: EMG response) is first determined for each of a plurality ofnerves, as follows. Referring to FIG. 1, a patient's vertebrae L1, L2,L3, L4, L5, and S1 are shown. In a preferred aspect of the presentinvention, a portion of the patient's cauda equina is stimulated (i.e.depolarized). This depolarization of a portion of the patient's caudaequina may be achieved by conducting a stimulus pulse having a knowncurrent level between an epidural stimulating electrode 11 and a patientreturn electrode 13. Electrodes 11 and 13 are referred to herein as“status” electrodes, as they assist in determining the initial status ofthe various nerve pathways). The epidural electrode is placed in theepidural space of the spine. Alternatively, the depolarization of aportion of the patient's cauda equina may be achieved by conducting astimulus pulse having a known current level between a pair of status(baseline) electrodes 12 and 14, which may be positioned adjacent the(thoracic/lumbar) T/L junction (above vertebra L1), as shown. Statuselectrodes 12 and 14 may be positioned in-line at the T/L junction, (asshown in FIG. 1). Status electrodes 12 and 14 could also be positionedon opposite lateral sides of the T/L junction.

In a preferred aspect, neuro-muscular (i.e., EMG), responses to thestimulus pulse by muscles coupled to nerves near the stimulatingelectrode are detected by electrodes positioned at each of a pluralityof myotome locations MR1, MR2, and MR3 on the patient's right leg, andmyotome locations ML1, ML2, and ML3 on the patient's left leg. Thesensing of neuro-muscular responses at these locations may be performedwith needle electrodes, or electrodes placed on the surface of thepatient's skin, as desired. An EMG response at each location MR1 to MR6is detected when the maximum peak-to-peak height of the EMG response tothe stimulus pulse is greater than a predetermined mV value (called“onset”). Accordingly, the current level required to elicit an onset EMGresponse is called the “onset” current level. As described below, thecurrent level of the stimulus pulse or signal applied to the electrode11 or electrodes 12, 14 may be incremented from a low level until anonset EMG response is detected for one or more of the myotome locationsMR1 to ML3.

It is to be understood that myotome sensing may be carried out at morethan the three distal locations illustrated on each of the patient'slegs in FIG. 1. Generally, as greater numbers of distal myotomelocations are monitored, a greater number of spinal nerves correspondingto each of these myotome locations can be individually monitored,thereby enhancing the present system's nerve detection ability over alarger section of the patient's spinal column.

It is also to be understood that the present invention can be easilyadapted to cervical or thoracic spinal applications (in addition to theillustrated lumbar application of FIG. 1). In this case an appropriateportion of the spinal column is depolarized and myotome-sensinglocations are selected according to the physiology of the associatednerves for portion of the spinal column of interest. In exemplaryaspects, therefore, preferred myotome-sensing locations may thereforeinclude locations on the patient's arms, anal sphincter, bladder, andother areas, depending upon the vertebrae level where the spinal surgeryis to be performed.

In a preferred aspect, the current level of the stimulus signalconducted between status electrodes 11 and 13 (or 12 and 14) isincrementally increased in a staircase fashion as shown in the currentstaircase of FIG. 2 from a low value until an onset EMG response isdetected at one or more myotome locations. In a preferred embodiment,onset EMG response peak-to-peak value is between 60 mV and 80 mV. (It isnoted, however, that depending on the location the stimulating electroderelative to the nerve corresponding to a myotome and the nervehealth/status, an onset EMG response may not be detected as the currentlevel is incremented from the lowest level to the highest level shown inFIG. 2.) In the illustrated exemplary aspect, the current level is shownas increasing from 4 mA to 32 mA, in eight 4 mA increments where thecurrent level is incremented until an onset EMG response is detected.The present invention is not limited to these values and other currentranges (and other numbers “steps” in the staircase) may also be used, asis desired.

At lower current levels, an onset neuro-muscular (i.e., EMG) responsesto the stimulus pulse may not be detected at each myotome ML1 to MR3location. However, as the current level of the stimulus signal isincrementally increased (i.e.: moving up the staircase, step-by-step),an onset neuro-muscular (i.e., EMG) response may eventually be detectedat each of the various myotome locations ML1 through MR3 for each of thesix associated spinal nerves. As noted whether an onset EMG response isdetected for myotome depends on the location of the electrode relativeto the corresponding nerve and the nerve status/health. For example,when a nerve is compressed or impaired, the current level required togenerate an onset EMG response may be greater than the similar,non-compressed nerve at a similar distance from the stimulatingelectrode. Accordingly, the onset neuro-muscular response for each ofthe various myotome ML1 to MR3 locations may be elicited at differentstimulus current levels due at least in part to the various individualspinal nerves being compressed, impaired, etc., and also due simply todifferences in the individual nerve pathway sensitivities.

For example, referring to the example illustrated in FIG. 1, a stimulussignal having an initial current level is conducted between electrodes11 and 13 (or between electrodes 12 and 14). The current level of thestimulus pulse is increased step-by-step according to the intensitystaircase shown in FIG. 2 until an onset EMG response is detected at oneor more selected myotomes. In particular, a response to the increasingcurrent level stimulus pulse is detected at each of the various myotomelocations ML1 through MR3. Because each of the spinal nerve pathscorresponding to the various myotome locations ML1 through MR3 may havedifferent sensitivities (as noted), different onset EMG responses may bedetected at the different onset current levels for different myotomelocations.

For example, Table 1 illustrates the current level required to elicit anonset EMG response for myotome location. As seen in Table 1, myotomelocation ML1 detected an onset EMG response to the stimulus pulse for acurrent level of 4 mA. Similarly, myotome MR2 detected an onsetneuro-muscular/EMG response to the stimulus pulse for a current level of24 mA, etc. Summarizing in tabular form:

TABLE 1 Stimulus Current Level at Which Onset EMG Response is Detected:ML1 - 4 mA MR1 - 16 mA ML2 - 16 mA MR2 - 24 mA ML3 - 20 mA MR3 - 12 mA

The above detected stimulus current levels may then be optionally scaledto correspond to stimulus staircase levels 1 through 8, with the maximumsignal strength of 32 mA corresponding to “8”, as follows, and asillustrated for each of Myotome locations ML1 to MR3, as shown in Table2 based on the levels shown in Table 1.

TABLE 2 Scaled Neuro-muscular Response Onset Values: ML1 - 1 MR1 - 4ML2 - 4 MR2 - 6 ML3 - 5 MR3 - 3

Accordingly, by depolarizing a portion of the patient's cauda equina andby then measuring the current amplitude at which an onset neuro-muscular(i.e., EMG) response to the depolarization of the cauda equina isdetected in each of a plurality of spinal nerves, (i.e.: at each of themyotome locations corresponding to each of the individual spinalnerves), a method for determining the relative neuro-muscular responsefor each of the plurality of spinal nerves is provided. As such, therelative sensitivities of the various spinal nerve pathways with respectto one another can initially be determined. This information mayrepresent the relative health or status of the nerves coupled to eachmyotome location where the stimulating electrode is approximately thesame distance from each of the corresponding nerves. For example, thenerve corresponding to myotome location MR2 required 24 mA to elicit anonset EMG response in the corresponding muscle. Accordingly, this nervemay be compressed or otherwise physiologically inhibited.

These respective stimulus pulse current levels at which an onsetneuro-muscular response is detected for each of myotome locations ML1through MR3 are detected may then be electronically stored (as aninitial “baseline” set of onset EMG response current levels). In apreferred aspect, these stored levels may then be used to perform nervedetection for a probe at a location other than the midline as will beexplained. As noted, once an onset neuro-muscular or EMG-response hasbeen detected for each of the myotome locations, it is not necessary toapply further increased current level signals. As such, it may not benecessary for the current level of the signal to reach the top of thecurrent level staircase (as shown in FIG. 2) (provided a response hasbeen detected at each of the myotome locations).

By either reaching the end of the increasing current amplitudestaircase, (or by simply proceeding as far up the staircase as isnecessary to detect a response at each myotome location), the presentsystem obtains and stores an initial “baseline” set of current levelonset values for each myotome location. These onset values may be storedeither as absolute (i.e.: mA) or scaled (i.e.: 1 to 8) values. As notedthese values represent the baseline or initial nerve status for eachnerve corresponding to one of the myotome locations. This baseline onsetcurrent level may be displayed as a fixed value on a bar graft of LEDssuch as shown in FIG. 8A or 8B. At a later point, the nerve status ofthe nerves corresponding to the myotomes may be determined again byapplying a varying current level signal to the midline electrodes. If aprocedure is being performed on the patient, the onset current level forone or more of the corresponding nerves may change.

When the onset current level increases for a nerve this may indicatethat a nerve has been impacted by the procedure. The increased onsetcurrent level may also be displayed on the bar graft for the respectivemyotome (FIG. 8A/8B). In one embodiment, the baseline onset currentlevel is shown as a particular color LED in the bar graph for eachmyotome location and the increased onset current level value is shown asa different color LED on the bar graph. When the onset current leveldecreases for a nerve this may indicate that a nerve has been aided bythe procedure. The decreased onset current level may also be displayedon the bar graft for the respective myotome. In a preferred embodiment,the decreased onset current level value is shown as a third color LED onthe bar graph. When the onset current level remains constant, only thefirst color for the baseline onset current level is shown on the bargraph. In one embodiment, a blue LED is depicted for the baseline onsetcurrent level, an orange LED is depicted for an increased (over thebaseline) onset current level, and a green LED is depicted for adecreased onset current level. In one embodiment when the maximumcurrent level in the staircase does not elicit an onset EMG response fora myotome, the baseline LED may be set to flash to indicate thiscondition. Accordingly, a clinician may periodically request nervestatus (midline stimulation) readings to determine what impact,positive, negative, or neutral, a procedure has had on a patient. Theclinician can make this assessment by viewing the bar graphs on thedisplay shown in FIG. 8 for each of the myotome locations.

The above determined initial set baseline neuro-muscular response onsetcurrent levels for each nerve pathway (myotome location) may then beused in the first (i.e.: nerve sensing) aspect of the present invention,in which a system is provided for detecting the presence of a spinalnerve adjacent to the distal end of a single probe 20, or either ofprobes 20 or 22. (It is to be understood, however, that the forgoingnerve status system (which may experimentally determine neuro-muscularresponse onset values) is an optional aspect of the present nervedetection system. As such, it is not necessary to determine suchrelative or absolute neuro-muscular response baseline onset currentlevels as set forth above prior to nerve detection. Rather, generallyexpected or previously known current onset levels may instead be usedinstead. Such generally expected or previously known current onsetlevels may have been determined by experiments performed previously onother patients.

In accordance with the first aspect of the present invention, nervedetection (performed as the surgical tool or probe is advancing towardthe operative site), or nerve surveillance (performed in an ongoingfashion when the surgical tool or probe is stationary has alreadyreached the operative site) may be carried out, as follows.

The first (nerve detection/surveillance) aspect of the invention willnow be set forth.

Returning to FIG. 1, a system is provided to determine whether a nerveis positioned closely adjacent to either of two probes 20 and 22. Inaccordance with the present invention, probes 20 and 22 can be anymanner of surgical tool, including (electrified) cannulae through whichother surgical tools are introduced into the patient. In one aspect ofthe invention only one probe (e.g.: probe 20) is used. In anotheraspect, as illustrated, two probes (e.g.: 20 and 22) are used. Keepingwithin the scope of the present invention, more than two probes may alsobe used. In one preferred aspect, probe 20 is an electrified cannula andprobe 22 is a “confirmation electrode” which can be inserted throughcannula/probe 20, as will be explained. Probes 20 and 22 may haveelectrified distal ends, with electrodes 21 and 23 positioned thereon,respectively. (In the case of probe 20 being a cannula, electrode 21 maybe positioned on an electrified distal end of the cannula, oralternatively, the entire surface of the electrified cannula mayfunction as the electrode).

Nerve detection is accomplished as follows. A stimulus pulse is passedbetween electrode 21 (disposed on the distal end of a probe 20) andpatient return electrode 30. In instances where a second probe (22) isalso used, a stimulus pulse is passed between electrode 23 (disposed onthe distal end of a probe 22) and patient return electrode 30. In oneaspect, electrodes 21 or 23 operate as cathodes and patient returnelectrode 30 is an anode. In this case, probes 20 and 22 are monopolar.Preferably, when simultaneously using two probes (20 and 22) thestimulus pulse emitted by each of electrodes 21 and 23 is multiplexed,so as to distinguish between their signals.

It should be understood that electrodes 21 and 23 could be replaced byany combination of multiple electrodes, operating either in monopolar orbipolar mode. In the case where a single probe has multiple electrodes(replacing a single electrode such as electrode 21) probe 20 couldinstead be bi-polar with patient return electrode 30 no longer beingrequired.

Subsequent to the emission of a stimulus pulse from either of electrodes21 or 23, each of myotome locations ML1 through MR3 are monitored todetermine if they exhibit an EMG response.

In a preferred aspect, as shown in FIG. 3, the intensity of the stimuluspulse passing between electrodes 21 and 30 or between 22 and 30 ispreferably varied over time. Most preferably, the current intensitylevel of the stimulus pulse is incrementally increased step-by-step in a“staircase” fashion. As can be seen, the current may be increased in ten0.5 mA steps from 0.5 mA to 5.0 mA. This stimulus pulse is preferablyincreased one step at a time until a neuro-muscular (i.e., EMG) responseto the stimulus pulse is detected in each of myotome locations ML1through MR3.

For myotome locations that exhibit an EMG response as a result of thestimulus pulse, the present invention then records the lowest amplitudeof current required to elicit such a response. Subsequently, thisstimulus level is interpreted so as to produce an appropriate warningindication to the user that the surgical tool/probe is in closeproximity to the nerve.

For example, in a simplified preferred aspect, the staircase of stimuluspulses may comprise only three levels, (rather than the 8 levels whichare illustrated in FIG. 3). If an EMG response is recorded at aparticular myotome location for only the highest level of stimulation(i.e.: the third step on a 3-step staircase), then the system couldindicate a “low” alarm condition (since it took a relatively high levelof stimulation to produce an EMG response, it is therefore unlikely thatthe tool/probe distal tip(s) are in close proximity to a nerve). If anEMG response is instead first recorded when the middle level ofstimulation (i.e.: the second step on the 3-step staircase) is reached,then the system could indicate a “medium” alarm condition. Similarly, ifan EMG response is recorded when the lowest level of stimulation (i.e.:the first step on the 3-step staircase) is reached, then it is likelythat the probe tips(s) are positioned very close to a nerve, and thesystem, could indicate a “high” alarm condition.

As can be appreciated, an important advantage of increasing the stimuluscurrent intensity in a “staircase” function, increasing from lower tohigher levels of current intensity is that a “high” alarm conditionwould be reached prior to a “low” alarm condition being reached,providing an early warning to the surgeon. Moreover, as soon as a “high”alarm condition is reached, the present invention need not continuethrough to the end (third step) of the staircase function. In preferredaspects, when the current level of the applied signal to the probe (20or 22) elicits an EMG response greater than the pre-determined onset EMGresponse, the current level is not increased.

In the above-described simplified (only three levels of stimulation)illustration of the invention, it was assumed that all nerves respondsimilarly to similar levels of stimulation, and the proximity (nervedetection) warning was based upon this assumption. Specifically, in theabove-described simplified (three levels of stimulation) illustration,there was an assumed one-to-one (i.e. linear) mapping of the EMG onsetvalue data onto the response data when determining what level ofproximity warning indication should be elicited, if any. However, in thecase of actual spinal nerve roots, there is not only a naturalvariability in response onset value threshold, but there is often asubstantial variation in neuro-muscular response onset values betweenthe nerve pathways caused as a result of certain disease states, such asnerve root compression resulting from a herniated intervertebral disc.

Accordingly, in a preferred aspect of the present invention, the initial“baseline” neuro-muscular EMG response onset value data set whichcharacterizes the relative EMG onset values of the various nerve rootsof interest, (as described above), is used to guide the interpretationof EMG response data and any subsequent proximity warning indication, asfollows.

Referring back to FIG. 1 and Table 1, the stimulation staircasetransmitted between electrodes 11 and 13 (or 12 and 14) resulted inmeasures neuro-muscular (i.e.: EMG) response onset values of 4, 16, 20,16, 24 and 12 mA at myotome locations ML1, ML2, ML3, MR1, MR2 and MR3,respectively. As can be seen, twice the intensity of current wasrequired to produce a neuro-muscular response at MR2 as was required toproduce a neuro-muscular response at MR3 (since “24” mA is twice as bigas “12” mA). Thus, the nerve pathway to MR3 is more “sensitive” than toMR2 (since MR3 is able to exhibit a neuro-muscular response at ½ of thecurrent intensity required to exhibit a neuro-muscular response at MR2).Consequently, during nerve detection, when electrode 21 or 23(positioned on the distal end of tool/probe 20 or 22) is positionedadjacent the nerve root affiliated with MR3, twice the currentstimulation intensity was required to produce an EMG response. Incontrast, when electrode 21 or 23 (on the distal end of tool/probe 20 or22) was positioned adjacent to the nerve root affiliated with MR2, thesame level of stimulation that produced a response at MR3 would notproduce a response at MR2.

In accordance with preferred aspects of the present invention, thesensitivities of the various spinal nerve pathways (to their associatedmyotomes) are incorporated into the nerve detection function of theinvention by incorporating the various neuro-muscular response onsetvalues, as follows.

A decision is made that either of electrodes 21 or 23 are positionedadjacent to a spinal nerve when a neuro-muscular response is detected ata particular myotome location at a current intensity level that is lessthan, (or optionally equal to), the previously measured or input EMGresponse onset value for the particular spinal nerve corresponding tothat myotome. For example, referring to myotome location ML1, thepreviously determined neuro-muscular response onset level was 4 mA, asshown in Table 1. Should a neuro-muscular response to the stimulus pulsebe detected at a current intensity level at or below 4 mA, this wouldsignal the operator that the respective probe electrode 21 (or 23)emitting the stimulus pulse is in close proximity to the spinal nerve.Similarly, the neuro-muscular response onset value for myotome locationML2 was determined to be 16 mA, as shown in Table 1. Accordingly, shoulda neuro-muscular response be detected at a current intensity level ofless than or equal to 16 mA, this would indicate that respective probeelectrode 21 (or 23) emitting the stimulus pulse is in close proximityto the spinal nerve.

In addition, as illustrated in FIG. 4, “high”, “medium” and “low”warning levels may preferably be mapped onto each stimulation staircaselevel for each myotome location. For example, the neuro-muscular onsetlevel for ML1 was 4 mA, corresponding to the first level of the 8-levelstatus electrode current staircase of FIG. 2. Thus, the first (4 mA)step on the staircase is assigned a “high” warning level. Level two (8mA) is assigned a “medium” warning level and level three (12 mA) isassigned a “low” warning level. Thus, if an EMG response is recorded atML1 at the first stimulation level, (4 mA), a “high” proximity warningis given. If a response is detected at the second level (8 mA), then a“medium” proximity warning is given. If a response is detected at thethird level (12 mA), then a “low” proximity warning is given. Ifresponses are detected only above the third level, or if no responsesare detected, than no warning indication is given.

Similarly, for ML2, with a onset value of 16 mA, (i.e.: the fourth levelin the status electrode current staircase sequence), the “high”,“medium” and “low” warning levels are assigned starting at the fourthstep on the status electrode current staircase, with the fourth stepbeing “high”, the fifth level being “medium” and the sixth level being“low”, respectively, as shown. Accordingly, if an EMG response isdetected for ML2 at (or above) the first, second, third, or fourthsurveillance levels, (i.e.: 4, 18, 12 or 16 mA), then a “high” warningindication will be given. For a response initially detected at the fifthlevel (i.e.: 20 mA), then a “medium” warning indication is given. If aresponse is not detected until the sixth level (i.e.: 24 mA), then a“low” warning indication is given. If responses are detected only abovethe sixth level, or not at all, then no indication is given. Preferably,each of myotome locations ML1 through MR3 are monitored at conditionsindicating “high”, “medium” and “low” likelihood of a nerve beingdisposed adjacent the surgical tool/probe.

As can be seen in FIG. 4, ten levels are shown for each of the myotomelocations, whereas the illustrated status electrode current staircasehas only eight levels. These optional levels “9” and “10” are useful asfollows. Should scaled level 8 be the minimum onset level at which aneuro-muscular response is detected, levels “9” and “10” can be used toindicate “medium” and “low” warning levels, respectively.

As explained above, the various neuro-muscular response current onsetlevels used in detection of spinal nerves may either have been eitherdetermined in accordance with the second aspect of the presentinvention, or may simply correspond to a set of known or expected valuesinput by the user, or pre-set into the system's hardware/software. Ineither case, an advantage of the present system is that differentneuro-muscular response onset value levels may be used whensimultaneously sensing for different nerves. An advantage of this isthat the present invention is able to compensate for differentsensitivities among the various spinal nerves.

As can be seen comparing the current intensities of stimulus electrodes11 and 13 (or 12 and 14) as shown in FIG. 2 (i.e.: up to 32 mA) to thecurrent intensities of probe electrodes 21 and 23 as shown in FIG. 3(i.e.: up to 5.0 mA), the current intensities emitted by probeelectrodes 21 and 23 are less than that of electrodes 12 and 14. Thisfeature of the present invention is very advantageous in that electrodes21 and 23 are positioned much closer to the spinal nerves. As such,electrodes 21 and 23 do not depolarize a large portion of the caudaequina, as do electrodes 12 and 14. In addition, the placement ofelectrode 11 in the epidural space ensures that the electrode is at arelatively known distance from the spinal nerves.

In an optional preferred aspect of the invention, if a neuro-muscularresponse (greater than the onset EMG response) is detected for all sixmyotome sensing locations ML1 through MR3 before all of the steps on thestaircase is completed, the remaining steps need not be executed.

Moreover, if it has been determined that a maximal level of stimulationis required to elicit an EMG response at a particular myotome sensinglocation, then only the top three stimulation levels need to bemonitored during the neuro-muscular response detection sequence. In thiscase, the top three monitored levels will correspond to “high”,“medium”, and “low” probabilities of the surgical tool/probe beingdisposed adjacent the a nerve. In another optional aspect, if any of themyotome locations do not respond to the maximum stimulation level (i.e.:top step on the staircase), they are assigned the maximum scale value(i.e.: a “low” warning indication).

Preferably, each of the spinal nerves monitored at myotome locations ML1through MR3 will correspond to nerves exiting from successive vertebraealong the spine. For example, as shown in FIG. 5, a main spinal nerve 50will continuously branch out downwardly along the spinal column withspinal nerve 51 exiting between vertebrae L2 and L3 while nerve 52passes downwardly. Spinal nerve 53 exits between vertebrae L3 and L4while spinal nerve 54 passes downwardly to L4. Lastly, spinal nerve 55will exit between vertebrae L4 and L5 while spinal nerve 56 passesdownwardly. As can be seen, neuro-muscular (i.e., EMG) responsemeasurements taken at myotome location MR1 will correspond to EMGsignals in spinal nerve 51, response measurements taken at myotomelocation MR2 correspond to EMG signals in spinal nerve 53, and responsemeasurements taken at myotome location MR3 correspond to EMG signals inspinal nerve 55.

In accordance with the present invention, the detection of aneuro-muscular (EMG) response, whether in accordance with the first(i.e.: nerve detection), or second (i.e.: establishing initial“baseline” neuro-muscular response onset values) aspect of theinvention, may be accomplished as follows.

Referring to FIG. 6, an illustration of the waveform characteristics ofa stimulus pulse and a corresponding neuro-muscular (EMG) response asdetected at a myotome location is shown. An “EMG sampling window” 200may be defined at a fixed internal of time after the stimulus pulse 202is emitted. The boundaries of window 200 may be determined by theearliest and latest times that an EMG response may occur relative tostimulus pulse 202. In the case of stimulation near the lumbar spine,these times are, for example, about 10 milliseconds and 50 milliseconds,respectively.

During EMG sampling window 101, the EMG signal may optionally beamplified and filtered in accordance with guidelines known to thoseskilled in the art. The signal may then be rectified and passed througha threshold detector to produce a train of pulses representing thenumber of “humps” of certain amplitudes contained in the EMG waveform. Are-settable counting circuit may then count the number of humps and acomparator may determine whether the number of pulses is within anacceptable range. By way of example only, the number of acceptablepulses for EMG responses elicited by stimulation in the lumbar spineregion may range from about two to about five. If only one pulse iscounted, then it is unlikely that a true EMG response has occurred,since true EMG waveforms are typically biphasic (having at least onepositive curved pulse response and one negative curved pulse response)resulting in at least two pulses. This pulse-counting scheme helps todiscriminate between true EMG waveforms and noise, since noise signalsare typically either sporadic and monophasic (and therefore produce onlyone pulse) or repetitive (producing a high number of pulses during theEMG sampling window).

In a further optional refinement, a separate noise-sampling window maybe established to remove noise present in the EMG responses to increasethe ability of the system to discriminate between true EMG responses andfalse responses caused by noise. The boundaries of noise sampling windoware chosen such that there is no significant change of a true EMG signaloccurring during the window. For example, it may be deemed acceptablethat one curved pulse of an EMG response may be comprised primarily ofnoise, but if more than one curved pulse of an EMG response is primarilycomprised of noise, an alarm would be triggered indicating that excessnoise is present on that particular channel.

In preferred aspects of the present invention, both the optional secondaspect of determining the neuro-muscular response onset values for eachof the plurality of spinal nerves and the first aspect of sensing todetect if a nerve is positioned adjacent to a surgical tool/probe arerepeated over time. Preferably, the sensing of whether a nerve ispositioned adjacent to a surgical tool/probe is continuously repeated invery short intervals of time, such that the operator can be warned inreal time as the surgical tool/probe is advanced toward the nerve. Thepresent system of determining the neuro-muscular response onset valuesfor each of the plurality of spinal nerves is also preferably repeated,and may be repeated automatically, or under operator control.

Typically, the above two aspects of the present invention will not becarried out simultaneously. Rather, when the neuro-muscular responseonset values are being determined (using electrodes 11 and 13 or 12 and14), the operation of probe electrodes 21 and 23 will be suspended.Conversely, when sensing to determine whether a nerve is positionedadjacent either of probes 20 or 22, the operation of stimulationelectrodes 11 and 13 or 12 and 14 will be suspended. A standardreference electrode 32 may be used for grounding the recordingelectrodes at the myotomes.

FIG. 6 depicts a particular exemplary embodiment of the presentinvention. Other embodiments are also possible, and are encompassed bythe present invention. Pulse generator 100 creates pulse trains of anappropriate frequency and duration when instructed to do so bycontroller 118. By way of example, the pulse frequency may be between 1pulse-per-second and 10 pulses-per-second, and the pulse duration may bebetween 20 μsec and 300 μsec. Pulse generator 100 may be implementedusing an integrated circuit (IC), such as an ICL75556 (Intensity) orgenerated by a software module. Amplitude modulator 102 produces a pulseof appropriate amplitude as instructed by controller 118, and maycomprise a digital-to-analog converter such as a DAC08 (NationalSemiconductor). The output of amplitude modulator 102 drives outputstage 103, which puts out a pulse of the desired amplitude. Output stage103 may comprise a transformer coupled, constant-current output circuit.The output of output stage 103 is directed through output multiplexer106 by controller 118 to the appropriate electrodes, either to status(baseline) electrodes 11 and 13, or to a combination of screw test probe109, probe electrode 21, 23 and patient return electrode 13. Impedancemonitor 104 senses the voltage and current characteristics of the outputpulses and controller 118 elicits an error indication on error display127 if the impedance falls above or below certain pre-set limits. Inputkeys 116 may be present on a front panel of a control unit of thepresent invention, as depicted in FIG. 8, to allow the user to shiftbetween modes of operation.

EMG inputs 128 to 138 comprise the six pairs of electrodes used todetect EMG activity at six different myotome locations. It will beappreciated that the number of channels may vary depending upon thenumber of nerve roots and affiliated myotomes that need to be monitored.A reference electrode 140 may also be attached to the patient at alocation roughly central to the groupings of EMG electrodes 128 to 138to serve as a ground reference for the EMG input signals. Electrodes 128to 140 may either be of the needle-type or of the gelled foam type, orof any type appropriate for detecting low-level physiological signals.EMG input stage 142 may contain input protection circuit comprising, forexample, gas discharge elements (to suppress high voltage transients)and/or clamping diodes. Such clamping diodes are preferably of thelow-leakage types, such as SST-pads (Siliconix). The signal is thenpassed through amplifier/filter 144, which may amplify the signaldifferentially using an instrumentation amplifier such as an AD620(Analog Devices). The overall gain may be on the order of about 10,000:1to about 1,000,000:1, and the low and high filter bands may be in therange of about 1-100 Hz and 500 to 5,000 Hz, respectively. Suchfiltering may be accomplished digitally, in software, or with discretecomponents using techniques well known to those skilled in the art. Theamplified and filtered signal then passes through rectifier 141, whichmay be either a software rectifier or a hardware rectifier. The outputof rectifier 146 goes to threshold detector 147 which may be implementedeither in electronic hardware or in software. The output of thresholddetector 147 then goes to counter 148 which may also be implemented byeither software or hardware.

Controller 118 may be a microcomputer or microcontroller, or it may be aprogrammable gate array, or other hardware logic device. Displayelements 120 to 127 may be of any appropriate type, either individuallyimplemented (such as with multicolor LEDs) or as an integrated display(such as an LCD).

What is claimed is:
 1. A method of determining at least one of a changein nerve status and a change in nerve proximity, comprising the stepsof: (a) applying a first stimulus signal to a stimulus electrodecommunicatively linked to a controller that directs application ofelectrical stimulus signals, said stimulus electrode being situated on amedical device and located a distance from a nerve; (b) detecting with asensor communicatively linked to said controller an EMG responsegenerated by a muscle physiologically coupled to said nerve in responseto said first stimulus signal; (c) increasing said first stimulus signaland repeating steps (a) and (b) until said controller determines saiddetected EMG response associated with said nerve reaches a predeterminedminimum EMG value representing an evoked muscle action potential; (d)identifying the level of said first stimulus signal required to obtainsaid predetermined minimum EMG value; (e) storing in a non-transitoryfashion on the controller the identified level of said first stimulussignal required to obtain said predetermined minimum EMG value; (f)repeating steps (a) through (d) with a second stimulus signal toidentify the level of said second stimulus signal required to obtainsaid predetermined minimum EMG value associated with said nerve; and (g)recalling on the controller the identified level of said first stimulussignal level and determining at least one of a change in nerve statusand a change in nerve proximity based on a comparison of said firststimulus signal level required to obtain said predetermined minimum EMGvalue and said second stimulus signal level required to obtain saidpredetermined minimum EMG value.
 2. The method of claim 1, wherein saidfirst and second stimulus signals each have a fixed current level. 3.The method of claim 1, wherein said medical device includes at least oneof a cannula, a pedicle probe, a needle, a catheter, RF ablation deviceand a medical laser.
 4. The method of claim 1, wherein said EMG responseis detected through the use of at least one EMG electrode.
 5. The methodof claim 4, wherein said at least one EMG electrode comprises at leastone of a needle electrode disposed within said muscle and a surfaceelectrode located above said muscle.
 6. The method of claim 1, whereinstep (c) includes the sub-steps of measuring a magnitude of saiddetected EMG response and comparing said magnitude with saidpredetermined minimum EMG value.
 7. The method of claim 6, whereinpredetermined minimum EMG value is a predetermined peak-to-peak voltage.8. The method of claim 7, wherein the predetermined peak-to-peak voltageis between 60 mV and 80 mV.
 9. The method of claim 1, wherein thecontroller includes a display and an indicator of said determined changein at least one of said nerve status and nerve proximity is displayedfor a user.
 10. The method of claim 9, wherein the display is an LCDdisplay.
 11. The method of claim 9, wherein the display is a series ofLEDs.
 12. The method of claim 9, wherein the display also displays anindication of the determined first stimulus signal level.
 13. The methodof claim 9, wherein the indication of said determined first stimulussignal level comprises a first color and the indicator of said change inat least one of said nerve status and nerve proximity comprises adifferent color.